The explosive growth of cloud computing and big data applications has fueled the expansion of datacenters. Scaling of datacenter networks to support such expansion with current electronic switches is challenging as the link rate increases to 100 Gb/s and beyond. State-of-the-art electronic switches such as Broadcom's Tomahawk has a throughput of 3.2 Tb/s. At high data rate, the radix (input port count) becomes limited. For example, the switch mentioned above has a radix of 32 at 100 Gb/s. A large number of switches are needed to connect the data center. The energy consumption, cost, and latency become serious issues. Photonics technology has been proposed to facilitate the scaling of the datacenters, reducing energy consumption and cost. Examples of such photonic-assisted data center networks are described in N. Farrington, et al., “Helios: a hybrid electrical/optical switch architecture for modular data centers,” ACM SIGCOMM Comput. Commun. Rev., Vol. 41, no. 4, pp. 339-350 (2011); A. Vandat, H. Liu, X. Zhao, and C. Johnson, “The emerging optical data center,” in Optical Fiber Communication Conference, pp. OTuH2 (2011); N. Binkert, et al., “Optical high radix switch design,” Micro IEEE, Vol. 32, no. 3, pp. 100-109 (2012); G. Porter, et al., “Integrating microsecond circuit switching into the data center,” in Proceedings of the ACM SIGCOMM 2013 conference on SIGCOMM, New York, N.Y., USA, pp. 447-458 (2013); H. Liu, et al., “Circuit Switching Under the Radar with REACToR,” in Proceedings of the 11th USENIX Conference on Networked Systems Design and Implementation, Berkeley, Calif., USA, pp. 1-15 (2014); S. Rumley, M. Glick, R. Dutt, and K. Bergman, “Impact of photonic switch radix on realizing optical interconnection networks for exascale systems,” Proc. of the IEEE Optical Interconnects Conference, pp. 98-99 (2014); A. S. P. Khope, A. A. M. Saleh, J. E. Bowers, and R. C. Alferness, “Elastic WDM crossbar switch for data centers,” Proc. of the 2016 IEEE Optical Interconnects Conference (OI), pp. 48-49 (2016).
Integrated-optics technology has matured to a point that it has become a strong candidate technology in optical-circuit switching systems. An integrated-optics system comprises one or more optical waveguides formed on the surface of a substrate, such as a silicon wafer, where the optical waveguides can be combined in myriad arrangements to provide complex optical functionality. Each “surface waveguide” includes a light-guiding core surrounded by cladding material that substantially confines the light signal it conveys. For several reasons, such as compatibility with CMOS electronics, availability of large-scale substrates and volume foundries, etc., integrated-optics systems based on the use of single-crystal silicon as the core material (referred to, herein, as “silicon photonics”) has become a dominant PLC technology—particularly for large-scale systems, such as high-port-count OXCs. Low-cost silicon photonic switches could potentially be revolutionary for datacenter networks. They eliminate optical-electrical-optical (O-E-O) conversions and greatly reduce the number of costly and power hungry high-speed data links. Recently, silicon photonic switches with a port count of 32 have been reported, as described in K. Tanizawa et al., “Silicon photonic 32×32 strictly-non-blocking blade switch and its full path characterization,” in OptoElectronics and Communications Conference (OECC) 2016; Dritan Celo, Dominic J. Goodwill, Jia Jiang, and et al., “32×32 Silicon Photonic Switch,” presented at the Optoelectronics and Communications Conference (OECC), 2016. Unfortunately, the optical loss of these switches is excessively high, around 16 dB for a 32×32 switch. Such high loss exceeds the loss budget of most fiber optic links, which prevent them from being used in data center networks.
Silicon photonic switches with significantly lower optical loss have been realized by incorporating micro-electro-mechanical systems (MEMS) switching mechanism, as disclosed, for example, by Han, et al., in “Large-scale silicon photonic switches with movable directional couplers,” Optica, Vol. 2, pp. 1-6 (2015) and by Seok, et al., in “Large-scale broadband digital silicon photonic switches with vertical adiabatic couplers,” Optica, Vol. 3, pp. 64-70 (2016), which is incorporated herein by reference.
Unfortunately, silicon-photonic systems of the prior art are normally characterized by significant polarization sensitivity, which gives rise to large differences in their propagation and insertion losses for different polarization modes—referred to polarization-dependent loss (PDL). Many applications, such as fast optical circuit switches for scalable and reconfigurable datacenter networks, among others, require very low PDL, however. As a result, silicon-photonic based optical switching systems have found little traction in such applications and their use has been substantially limited to single-polarization systems.
Attempts to reduce the polarization dependence of silicon photonic switches include approaches such as those disclosed by Nakamura, et al., in “High extinction ratio optical switching independently of temperature with silicon photonic 1×8 switch,” in Optical Fiber Communication Conference, 2012, p. OTu2I-3, wherein polarization sensitivity is mitigated by the use of a thick (1.5 micrometer) rib waveguide. It should be noted, however, while the use of thick waveguides affords reduced polarization sensitivity, it is achieved at the expense of lower integration density. Furthermore, the systems disclosed by Nakamura et al rely up on a switch architecture comprising cascaded stages of 1×2 and 2×2 Mach-Zehnder interferometers, which gives rise to high cumulative losses for large-port-count arrangements.
Other attempts to realize polarization-insensitive switches in the prior art include those disclosed by K. Tanizawa, et al., in “4×4 Si-wire optical path switch with off-chip polarization diversity,” Opto-Electronics and Communications Conference (OECC), 2015, 2015, pp. 1-3, which uses polarization splitters and combiners. In these systems, incoming signals are first split into its transverse-electric (TE) and transverse-magnetic (TM) polarization components, which propagate through two separate sets of switches. Unfortunately, such systems required the use of external (off-chip) polarization splitters/combiners, which are bulky, or directional couplers as on-chip polarization splitters and combiners; however, it still requires two sets of switches and complex crossing waveguides to route the TE and TM signals. In some cases, external polarization splitters/combiners feed the split signals to opposite ends of the switch with complementary ports, as disclosed in K. Tanizawa, et al., “Non-duplicate polarization-diversity 8×8 Si-wire PILOSS switch integrated with polarization splitter-rotators,” Opt. Express 25,10885-10892 (2017), which reduces the number of switches to one set but at the expense of higher crosstalk. Most importantly, all such prior-art polarization-diverse switches use many stages of cascaded 2×2 switches, which leads to high insertion loss at high port count, thus limiting their scalability to high radix switches.
A second issue of silicon photonic switch is size of the switch known as the port count or radix of the switch. Silicon photonic switches are manufactured using optical lithography in integrated circuit foundries. The maximum area that can be exposed in standard lithography systems (steppers or scanners), called reticle field, is limited to a few centimeters by a few centimeters. This limits the number of switching cells that can be integrated in one reticle field and therefore the maximum port count (radix) of the switch. Large-scale switches are needed, for example in hyperscale data centers, to connect a large number of nodes.
A practical, fast, low-cost, low-loss optical switching technology that has low polarization-dependent loss and scalable to high port count is, as yet, unavailable in the prior art.